So I have to vaccinate my sheep with Cydectin in the next day or so. It has to be put under the skin just below the ear. How I am going to do this I have no idea as I am not very experienced at things like this. Any helpful tips or advice? It doesn't help that the sheep are only a week or two away from lambing!

politely ask the patient to stay still for a moment as they are sheep that is not my thing.

use the recommended hygiene protocols
pinch up the relevant flap and having gently turned it towards you position the needle under your thumb.
holding the barrel of the syringe firmly with thumb and two fingers of one hand roll and lift your pinch of the other hand a little to tighten the near side and relax the far side, insert needle though the nearside skin at an angle to miss all underlying tissues. a firm fast short controlled jab usually works but with a good needle and the softish nature of behind the ear skin it should not take much force. with the proper needle it should be fairly easy not to go though and out the other side ( if it happens pull back until inside again and continue as below )
deliver dose using two fingers and thumb on the handles and plunger ( practice handling the syringes so as swapping from barrel grip to inject dose grip is easy ) and remove the needle by pulling out in a strait line
apologise to patient for the indignity and explain it is for their own good

practice on a chicken leg sheep are a little tougher of skin but it will help with the general mechanics.
with practice doing a SCI will be the easier for both parties once the chicken leg is ok with the procedure tis round up time.

Dpack I love your response If only..... !! I intended doing it today but things didn't go to plan so its now pencilled in for tomorrow. Could be interesting as I will be trying to do it all by myself. I have injected before, its the site that worries me a bit as I have visions of poking them in the jugular or something silly like that...